Tony Blair has promised to look again at the 300-year-old law banning Roman Catholics and other non-Protestants from succession to the British throne.

In an interview with the Glasgow-based Herald newspaper, the prime minister acknowledged that the 1701 Act of Settlement was "plainly discriminatory" and promised to re-examine it if Labour were elected to a second term.

The Act of Settlement is a cornerstone of the British constitution and any move to repeal or amend it would represent one of the greatest constitutional changes in three centuries.

Mr Blair said: "Obviously, in principle, it can't be right that Catholics are unable to succeed, so we will examine it again."

He added: "It is certainly worth looking at, because it is plainly discriminatory."

The Guardian is currently backing a legal challenge to the act on the grounds that it clashes with the Human Rights Act and should be reinterpreted or removed from the statute book.

Although there is support for repeal from members of all the main political parties - and the Scottish parliament has unanimously passed a motion calling for it to go - the government has previously said that this would be too complicated.

Mr Blair yesterday acknowledged the difficulties in removing the act form the statute book - not least the possibility that it would result in the disestablishment of the Church of England - but said it was time to look at the issue afresh.